High purity metals and alloys are used in the upgrading and processing of nuclear fuels and containment of nuclear reactor fuel rods. These same metals and their alloys are used in the containment of the actual reactors both for power generation and for military vessels such as submarines. These same metals during the course of their service come under constant high radiation doses and accumulate low to moderate levels of radioactivity within the metal itself. This service-acquired inherent radioactivity is termed “volumetric radioactivity,” as opposed to surface radioactivity which is due to radioactive particles on the surface of the metal form or part. The surface radioactivity can be washed or scoured off, but most of the volumetric radioactivity cannot be removed by physical processes such as washing or heating even up to the vary high melting point of the metal or metal alloy. Nickel and its alloys, for example, are widely used for these applications and accumulate various levels of volumetric radiation due to technetium 99 (a beta radiation emitter) and other radioactive isotopes. These radioactive isotopes include both gamma and beta emitters which are of concern along with the weaker alpha emitters. The removal of the volumetric radioactive isotopes from the volumetrically radioactive contaminated nickel and its alloys is of significant importance in order to recover this valuable metal.
Since nickel is the preferred containment material for nuclear processing and reactors, volumetrically radioactive contaminated nickel will continue to be generated. The reuse of this contaminated nickel by re-melting it and restricting it only for nuclear use can be done but there is a quantity mismatch since there is a very large inventory of this volumetrically contaminated nickel already being stockpiled awaiting some means of purifying it back to background levels and for reuse in commercial applications. This contaminated nickel at its current volumetric radioactive levels cannot be recycled back into the normal commercial metal alloying and fabrication processes. As older nuclear reactors and older nuclear fuel processing facilities are decommissioned, such contaminated nickel and nickel alloys continue to be generated. Not only is the economic loss and value of major concern, the necessity to recycle and reuse the limited amount of metals such as nickel is a worldwide concern in order to maintain the long term availability of such important metals.
Various decontamination processes are known in the art, specifically for decontamination of nickel. Nickel can be recovered by selectively stripping from an acidic solution by electrowinning. See U.S. Pat. No. 3,853,725. Nickel may also be removed by liquid, liquid extraction or solvent extraction. See U.S. Pat. Nos. 4,162,296 and 4,196,076. Further, various phosphate type compounds have been used in the removal of nickel. See U.S. Pat. Nos. 4,162,296; 4,624,703; 4,718,996; 4,528,165 and 4,808,034.
U.S. Pat. No. 7,988,937 to Smith et al discloses a method for the volumetric decontamination of radioactive material in which the metals are recovered in the form of metal hydroxides. The present application provides an improvement in costs and efficiency by the use of different oxidative conditions and by recovering metal carbonates or hydroxides.
It is known that metallic nickel, contaminated with fission products, can be decontaminated to remove any actinides present by direct electro-refining based on the differences in reduction potential in the electromotive force (emf) series. Actinide removal is favored by two phenomena during electro-refining. Actinides have a significantly higher reduction potential relative to nickel and they are normally won from molten salt electrolyte rather than from aqueous electrolyte. See U.S. Pat. Nos. 3,928,153 and 3,891,741, for example. The prior art methods of decontaminating metals with surface radioactivity cannot be used for the removal of volumetric radioactive isotopes.
In spite of these disclosures, there remains a need for an economical and efficient method of decontaminating radioactive metals.